In a cellular communications system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
In a Code Division Multiple Access (CDMA) mobile communication system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band. In other words, individual radio "channels" correspond to and are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as Applications of CDMA and Wireless/Personal Communications, Garg, Vijay K. et al., Prentice-Hall 1997.
Spread spectrum communications permit mobile transmissions to be received at two or more ("diverse") base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another antenna sector connected to the same base station), without any perceptible disturbance in the voice or data communications. This kind of handover is typically called diversity handover.
During diversity handover, the signaling and voice information from plural sources is combined in a common point with decisions made on the "quality" of the received data. In soft handover, as a mobile station involved in a call moves to the edge of a base station's cell, the adjacent cell's base station assigns a transceiver to the same call while a transceiver in the current base station continues to handle that call as well. As a result, the call is handed over on a make-before-break basis. Soft diversity handover is therefore a process where two or more base stations handle the call simultaneously until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call. "Softer" diversity handover occurs when the mobile station is in handover between two different antenna sectors connected to the same, multi-sectored base station using a similar make-before-break methodology.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user's communication interferes with the communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station's cell boundary. As a result, distant mobile communications are overshadowed and dominated by close-in mobile stations which is why this condition is sometimes referred as the "near-far effect."
The physical characteristics of a radio channel vary significantly for a number of reasons. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. If the transmission power of a mobile station signal is too low, the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of the signals adds to the delay associated with radio access procedures, increases data processing overhead, and reduces the available radio bandwidth because erroneously received signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system. Ideally, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of their distance from the base station.
Interference is a particularly severe problem in CDMA systems because large numbers of radios transmit on the same frequency. If one mobile station transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of signals received from other mobile radios to the point that a receiving base station cannot correctly demodulate transmissions from the other mobile radios. In fact, if a mobile station transmits a signal at twice the power level needed for the signal to be accurately received at the base station receiver, that mobile signal occupies roughly twice the system capacity as it would if the signal were transmit at the optimum power level. Unregulated, it is not uncommon for a strong mobile station to transmit signals that are received at the base station at many, many times the strength of other mobile transmissions. The loss of system capacity to such excessively "strong" mobile stations is unacceptable.
Additional problems are associated with transmitting with too much power. One is the so-called "party effect." If a mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can "be heard" compounding the already serious interference problem.
Another problem is wasted battery power. It is very important to conserve the limited battery life in mobile radios. By far, the largest drain on a mobile's battery occurs during transmission. A significant objective for any power control approach, therefore, is to reduce transmit power where possible without increasing the number of retransmissions to an unacceptably high level as a consequence of that power reduction. Except for battery consumption, the above-described problems with setting transmission power also apply to downlink radio transmissions from base stations.
Transmit power control (TPC) is therefore important in any mobile radio communications system, and is a particularly significant factor in improving the performance and capacity of a CDMA system. In uplink TPC, the mobile station attempts to control its transmit power based on the power control messages sent to the mobile station from the base station with the goal of controlling the power level of signals received at the base station within a relatively small tolerance, e.g., 1 dB for all mobile station transmissions received at that base station.
More specifically, transmit power control strives to keep the received carrier-to-interference ratio (CIR) close to a target CIR. Alternate measures of signal quality may also be used such as received signal-to-interference ratio (SIR), received signal strength (RSSI), etc. The carrier-to-interference ratio actually received at a base station or mobile station depends on the received carrier power and the current interference level. Received carrier power corresponds to the transmit power level P.sub.tx minus the path loss L. The path loss L may also be represented as a negative gain. Such a gain factor includes two components for a radio channel: a slow fading gain G.sub.s, and a fast fading gain G.sub.f. The interference from other users in the CDMA system also depends on the spreading factor employed by other transmitters. Accordingly, the carrier-to-interference ratio may be roughly determined in accordance with the following: ##EQU1##
where P corresponds to the transmit power level, G corresponds to the path gain (including both fast and slowing fading components), SF is the spreading factor which is equal to the number of "chips" used to spread a data symbol, and N is the background noise.
The power related issues described above for uplink (or reverse) power control for transmissions from the mobile station to the base station also apply in the downlink (or forward) transmit direction from a transceiver in the base station to the mobile station. In downlink power control, the base station varies the power of the transceiver transmitting to the mobile station depending on downlink transmit power control messages or commands sent by the mobile station.
Because power control in CDMA systems is very important, transmit power control adjustments often occur very frequently, e.g., every 0.625 milliseconds. During transmit power regulation, each station (mobile and base) continually measures the transmit power level received from the other station and determines whether the measured value is greater than a reference value. If so, a transmit power control bit having one value is sent from one station instructing the other station to decrease its transmit power by a predetermined increment, e.g., 1 dB, down to a minimum transmit power value. On the other hand, when the measured value is less than the reference value, the transmit power control bit(s) with the opposite value(s) is (are) transmit to increase transmit power by a predetermined increment, e.g., 1 dB, up to a maximum value. Because power control commands occur very frequently, it is desirable not to use large numbers of bits to avoid increasing the signaling "overhead."
Various factors may cause the received carrier-to-interference ratio to differ from a target carrier-to-inference ratio by as much as 10 dB or more. These factors include environmental conditions such as a rapidly varying radio channel, changing temperatures which affect the performance of radio equipment, practical implementation limitations (e.g., non-linear components used to construct base and mobile stations), and delays in power control commands to name a few. One way to approach power control in view of such problems is to employ to employ an open loop power control in combination with a closed loop power control. In open loop power control, the transmit power is calculated at the transmitter based on one or more parameters, and the calculated value is used to set the transmit power level. The transmit power may be adjusted in order to match an estimated path loss so that the signal is received at the base station at a predetermined power level. Closed loop power control relies on feedback from the receiver so that the transmitter knows, for example, at what CIR level the transmitted signal was received. Using this feedback, the transmitter then appropriately adjusts its transmit power level. A drawback with this approach is its complexity in that two types of power control must be implemented and coordinated. It is often difficult to guarantee that the two power control schemes work together harmoniously and special hardware may be needed to "mix" these two types of power control. Another drawback is that since this approach responds to path loss changes, it does not compensate for changes in interference.
Another less complex approach is a power "ramping" power control technique such as described in Ericsson's U.S. Pat. No. 5,430,760 to Dent. The mobile station initiates a random access at a low initial transmit power level and gradually (e.g., incrementally) increases the transmission power level until the base station detects and acknowledges the access signal. Once detected, the power level of the message is maintained at the detected level.
While both of these approaches are useful, neither is optimum in all situations and in all respects. As can be seen from equation (1), the interference from other users depends to a significant extent on the spreading factor employed by that user. A low spreading factor corresponding to a smaller number of chips per symbol increases the interference generated by user i considerably. Consider the following scenario. A mobile user, having a low spreading factor or otherwise transmitting at a high power, is traveling through a city with a number of buildings and other obstacles. The serving base station is relatively far way. However, as the mobile user rounds a street corner, the user is suddenly very close to another base station previously shadowed or blocked by that building. One practical effect is that when this mobile transmitting at high power rounds the corner, it "blasts" the new, closer base station and nearby users currently being served by that base station. The net result is a large, unnecessary increase in interference in the new base station's cell(s) which lowers the carrier-to-interference ratio for the other mobile users in the cell(s). Consequently, those other mobile users will increase their transmit power levels in order to maintain a reasonable carrier-to-interference ratio, i.e., the party effect referred to above.
Another example concerns mobile data users that employ low spreading factors. Such users typically do not significantly increase the interference level as long as their data sending/receiving activities are low. However, should such low spreading factor data users start transmitting at a high data rate, that transmission will be at a much larger transmit power suddenly increasing the interference level. If a 1 dB stepsize is employed to decrease that user's transmit power, other users in that cell will not be able to raise their output power fast enough to compensate for this new situation.
What is needed, therefore, is an effective power control mechanism that quickly decreases the power of such a mobile user. Such a power control mechanism should also preferably raise power quickly as well but more restrictively than when decreasing power. One approach is to vary the step size in the incremental power control approach mentioned above to accommodate both large and small step sizes. Normally, a relatively small step size is employed. But in situations like that just described where a high power transmission mobile rounds a corner, a large power decrease step is necessary to reduce that mobile's power quickly and by a significant amount. A variable step size also addresses problems related to rapid fading of a radio channel where a mobile is traveling at high speed. Quickly changing fading conditions of the radio channel mean that the transmit power to and from that mobile terminal must be adjusted rapidly using variable step sizes when such changes are detected. Even so, for a fast moving mobile user, it still may be quite difficult to compensate for fast fading. In that case, a 1 dB power step size may be too large or will only serve to increase power fluctuations, and it may be better to use small size power steps in this situation.
A drawback with sending variable step size power control commands is added overhead. In order to compensate for quickly changing transmission conditions, the variable step sizes must be transmit very frequently. In the example where a TPC command is sent every 0.625 msec time slot, a variable step size value is transmit 1,600 times per second. When frequently transmitted, variable step size commands enable the transmit power control to track fast channel fading and other abrupt changes in transmission condition relatively well. However, there is a need to reduce undesirable signaling overhead associated with sending so much step size data and the associated loss of useable radio bandwidth for user traffic.
It is an object of the present invention to overcome the problems identified above, and in particular, to provide both efficient and effective power control in a mobile communications system.
It is an object of the present invention to achieve a power control technique that adapts to rapidly changing radio transmission conditions and situations.
It is an object of the present invention to provide an adaptive power control technique that ensures a satisfactory quality of communication at a minimum level of interference.
It is an object of the present invention to provide an adaptive power control technique with a minimal amount of control signaling overhead.
The present invention overcomes the identified problems and meets these and other objectives by efficiently and effectively controlling the transmit power of a radio transceiver. The value of a signal parameter detected from a signal received by the radio transceiver is compared with a desired signal parameter value, and a difference is determined. A transmit power control command is sent to the radio transceiver and may instruct, for example, an increase or decrease in the level of radio transmit power. Associated with the transmit power control command is a power control indicator indicating which type of power control adjustment should be used by the radio transceiver depending upon the determined difference. For example, one or more flag bits may accompany the power control command. Depending on a number of indicator bits employed, many different power control adjustments may be employed.
Other types of indicators with low overhead may also be employed. For example, different power control command bit patterns may be used. One pattern corresponds to a first type of power control adjustment and another pattern corresponds to another type of power control adjustment. Different power control adjustment type messages may also be conveyed using other, non-power related control signaling messages frequently exchanged between the base and mobile stations. Moreover, any message that is sent in the normal operation and/or control of the base and mobile stations may be used to convey power control adjustment type messages without significantly adding to the overhead.
In one example embodiment, the power control indicator includes a single flag bit. A first value indicates that a first type of power control adjustment should be used; the second value indicates that a second type of power control adjustment should be used. In any event, the power control indicator itself does not include specific details of the first or second type of power control adjustment. Because only the indicator is sent (and not the details), signaling overhead and bandwidth consumption related to frequently sent transmit power control commands are kept to a minimum. The details of the first and second power control adjustments are initially stored in the radio transceiver. Such details may be updated when desirable, but the frequency of such updating is likely to be infrequent.
The first and second type of power control adjustments may include a first and second power adjustment step size, where one step size might be used in one type of power adjustment situation and another step size might be used in another type of situation. Alternatively, the first and second type of power control adjustments might correspond to two different power control schemes for adjusting the transmit power of the radio transceiver. The invention may be implemented for "uplink" power control in a radio network node with the radio transceiver corresponding to one or more mobile stations. In addition, the invention may be implemented for the "downlink" direction in a mobile station with the radio transceiver corresponding to a base station in the radio network.